spiceworks will do inventory for a site. not sure if you could really put it on a laptop and haul it around or if it needs to be on a server and staying at a customer site. havent used it much, but it will sort of do what you want, and its free.
usually for our customers we use visio...and i god damn hate it for that sort of thing. great for connectivity drawings or physical layouts, but my company uses it for inventory and password management...poorly.
The Dude will also do some scanning, but its nothing complex or special as far as doing inventory...just network scans, iirc.
if you used manage antivirus that will do inventory for devices it is installed on with some clients, like symantec endpoint protection or mcafee epolicy console.
novell still has zenworks for asset management (ugh)...and you can look into solar winds, but all of that is going to cost money, probably per site and/or client.
now, if you want something you can sell tha twill do some of this, but remain affordable, look into the cicso Onplus
http://www.cisco.com/en/US/products/ps11792/index.html
we just started testing a unit. im not clear on the details yet but one of my co-workers was telling me about it. i know its relatively cheap for each device/service, will do network monitoring and inventory and give you remote access anywhere you have an internet connection. not sure that it will keep up with passwords and other data like that but that sort of information is probably best kept encrypted anyways.